little bonding clay and none if possible, by depending upon the use of organic binders and the sintering together of the particles upon reheating the molded articles.
Refractories of this type show considerable deformation under pressure at higher temperatures and hence are not suited for use under these conditions, even though the fusion point is high in itself, approaching 2,000°C. Fused alumina products consist principally of laboratory ware and furnace parts though various larger shapes are also made for heat-resisting purposes. Refractory cements for imbedding heating coils are made from fused alumina and clay. Bricks made from bauxite with some admixture of clay have been found by Kanolt to fuse at 1,820°C. The specific gravity of "alun-
Si 02 AlgSiOs AlgOj
FIG. 10.—Silica-alumina fusion diagram.
dum " ware is given at 3.91, the specific heat at 0.19 at low temperatures, the thermal conductivity at 2.1 times that of firebrick and the coefficient of expansion as being 0.0000071 per degree Centigrade.
Sillimanite Type.—As the silica content of alumina mixtures increases the fusion point decreases until the formation of a compound is indicated at the point C of the curve. This point corresponds to the composition A^Os.SiC^ of the mineral sillimanite and to a temperature of 1,816°C. Such mixtures may also be prepared by blending kaolin and anhydrous alumina in the proportion, by weight, of 258 :102 and sintering or fusing this material, in a forced draft or electric furnace. Sintering of the mass down to practically constant volume can be accomplished by heating to about 1,525°C. The addition of 2 per cent of boric acid or of an equal amount of magnesia makes it possible to reduce the temperature to approximately 1,480°C. Microscopic examination should prove the mass to be largely crystalline sillimanite. By suitable grinding and the use of organic binders, such as dextrin, gum tragacanth, linseed oil, etc., the calcine may be worked up into suitable shapes which must be refired to at least 1,450°C.
The product thus made possesses a well-defined melting point, approaching 1,816°C., according to the degree of purity, and is able to resist considerable pressure at furnace temperatures with but little deformation.